There are a couple of ways to get them off. First, I would secure the valve stem with some forceps coated in latex, or something safe like that. This way the stem will not spin. Now, with your cams removed, pull up on the closing shim and twist it a little bit to re-seat the half rings a bit and offer a new smoother avenue of removal. Be sure you have your opener shims removed and your opener rockers are out of the way...I would assume this is done since you've removed the cams. Now, gently, put the end of a smaller diameter end driver or phillips screw driver (or your finger on the testastrettas) through gap behind the closing rocker and use it to pull the closer rocker forks down the shaft so it is plenty far away from the seat of the shim. Now try to twist and force the closer down from its location on the stem. Sometimes gently snapping the closer shut on the seat of the shim helps to jar it loose. If this proves to be too difficult, I would gently take a deep well socket and rubber mallet and place it over the stem and shim and VERY gently tap it down. Be sure to have a magnetic pickup ready to aid with the removal of the half-rings. If you have MBP collets, I would highly recommend the socket method. All in all this isn't that tough. Be sure to use some particle free grease (no graphite, molybdenum, etc) to hold the rings flush to the stem upon reinsertion- this allows them to stay put. I like to use a dab or red engine pre-lube since it dissolves well in oil. Good luck!!
Forceps, extension cord, or fray-free nylon rope is ok to put into the spark plug hole to hold the valves up. Be careful not to force the piston into the valves.